1. Field of the Invention
The invention relates to semiconductor wafer processing systems.
2. Description of the Background Art
Semiconductor processing systems that perform "dry" etching of semiconductor wafers via plasmic gases, also known as reactive ion etching (RIE) require constant monitoring. While it is possible to predefine the etch parameters and allow the systems to perform the etch process unmonitored, conditions within the systems can change over time. Minute changes in the composition or pressure of an etch gas or process chamber or wafer temperature creates undesirable etch results.
For example, DRAM memory circuits are fabricated from semiconductor wafers using deep trench technology. A single DRAM memory cell consists of a capacitive storage cell and a switching element (i.e., a MOSFET transistor). Information (in the form of electrical charge) stored in the cell is passed on to other circuitry when the switching element is activated. Essentially very deep (on the order of 3-20 .mu.m) channels or trenches must be formed in a semiconducting substrate in order to create the capacitive storage cells. Otherwise, the information is not sustained (i.e., the electrical charge "leaks out" of the storage cell).
Such trench etch circuits are formed by etching away different layers of insulating material deposited upon the substrate and the substrate itself in various steps. For example, first a photoresist mask is placed over an insulating layer or film (silicon dioxide or other similar material). The mask contains a desired circuit pattern to be etched into the insulating layer. It is important that etching of the insulating layer stop at the point where the substrate (silicon or other similar composition) is first revealed at the bottom of the trench. In a next step, the remaining portion of the photoresist mask is removed via an ashing operation so as to not remove any of the remaining insulating film or improperly etch the substrate. In a next step, a more involved chemical process etches a trench into the substrate material while continuously redepositing the insulating layer material so as to not attack the original insulating layer defining the circuit pattern. It can easily be seen that if the etch process during any one step exceeds the predetermined endpoint, the substrate, insulating layer and/or resultant circuit pattern may be damaged. As such, these systems rely upon some type of in situ measurement to determine the progressive depth of the etch process. In situ measurement provides greater control of the etch process and improves uniformity over a batch of processed wafers.
There has been some success in the art of developing in situ etch depth measuring systems that utilize optical emission spectroscopy to monitor light emissions from the plasma as the etch process progresses. One such system is disclosed in U.S. Pat. No. 5,308,414 to O'Neill et al. Such a system monitors the optical emission intensity of the plasma in a narrow band as well as a wide band and generates signals indicative of the spectral intensity of the plasma. When the signals diverge, a termination signal is generated thereby terminating the etch process. Other techniques include the use of laser interferometry, beamsplitters and diffraction gratings to measure the phase shift of a laser beam reflected from two closely spaced surfaces. For example, the phase shift between a first beam reflected off the mask pattern and the beam reflected off an etched portion of the wafer is measured and compared to a predetermined phase shift that corresponds to the desired etch depth. Unfortunately such monitoring and measuring systems are plagued by inadequate signal to noise ratios. Additionally, the minimum etch depth is limited by the wavelength of the light source used in the monitor. Another technique for measuring etch depth is ellipsometry, which measures the change in polarization of light upon reflection of the light from a surface. Unfortunately, the error in etch depth detection in systems that use polarized laser beams is too great to be useful.
In situ etch depth monitoring is of particular interest in systems where plasma excitation coils are used. Such a system is the Decoupled Plasma Source (DPS) system manufactured by Applied Materials, Inc. of Santa Clara, Calif. For example, RF power applied to a coil configuration atop a process chamber assists in creating the plasma that performs the etch process. However, the RF power may inductively couple into the neighboring monitoring equipment thereby corrupting the monitoring signals. As such, in situ monitoring of etch depth in a high power RF environment is inadequate and prone to severe inaccuracy.
Therefore, a need exists in the art for an apparatus for performing direct, in situ measurement of etch depth in a high power RF environment as well as monitoring other processes performed by a semiconductor wafer processing system.